Home equity lines of credit (HELOC) are revolving loans, like credit cards, that are backed by the value of a borrower's home. The credit line tends to be available to the borrower for a fixed amount of time, generally ten years. At the end of that period, the line can sometimes be renewed, or the borrower may need to pay back any outstanding balance.

This chart from Experian shows the amount of money Americans borrowed against their homes in each year going back to 2000.

The fact that outstanding HELOC balance levels are returning to housing bubble levels tells Gundlach that consumers may be entering a borrowing frenzy.